The aim of this project is to learn more about the mechanism of neutralization of cross-reactive immune responses to the influenza virus surface glycoproteins, hemagglutinin and neuraminidase, and to investigate the quantitative contribution of these mechanisms to protection. To approach this we will map the epitopes of a large panel of human (in collaboration with Project 2) and mouse cross-reactive anti-neuraminidase antibodies in order to define broadly neutralizing epitopes. Further, efforts will be made to characterize the quantitative contribution of different mechanisms of neutralization of a vast panel of broadly reactive antistalk and anti-neuraminidase monoclonal antibodies (of human and mouse origin, in collaboration with Project 2) and polyclonal sera. This work will involve in vivo imaging as well as a panel of assays that measure inhibition of viral entry/fusion, viral egress, hemagglutinin maturation, neuraminidase inhibition, complement activation and others (Project 3 will focus on antibody-dependent cell-mediated cytotoxicity). Finally, we use recombinant chimeric hemagglutinin and neuraminidase expressing influenza B viruses to study the impact of polyclonal anti-stalk and anti-neuraminidase responses on influenza virus pathogenesis and transmission in the mouse, guinea pig and ferret model.